This invention relates to an improved colloidal graphite hot forming lubricant and to a method of hot forming powdered ferrous preforms using this lubricant.
In commercial production hot forming operations, a press is used to mechanically work a hot metal preform into predetermined shape. A lubricant is applied to the metal preform as well as to the die components in the press. Ferrous powdered metal articles are hot formed at temperatures of about 1200.degree. - 2000.degree. F. Colloidal graphite dispersions are effective lubricants at these temperatures, and are typically used as hot forming lubricants.
There is a high interest in replacing wrought metal parts with parts derived from powdered metal preforms. Properties such as ductility and impact strength of hot formed articles made from powdered metal preforms benefit greatly from a sintering of the preform (commonly referred to as presintering by those skilled in the art) prior to the hot forming operation. In general, the higher the presintering temperature and the longer the soaking time at this temperature, the greater the benefit. This preliminary heat treatment, presinter, also produces carbon solution and oxide reduction, the latter being of particular benefit to impact strength.
A normal and accepted presintering procedure for producing maximum properties in ferrous powdered metal articles involves furnace heating a ferrous powdered metal preform for about one hour at about 2000.degree. F. This actually requires a total furnace time of several hours. After presintering, the preform is cooled to about 400.degree. F. for lubricant application. A colloidal graphite lubricant is then applied, for example by dipping the preform into it. Then the preform is reheated to the desired hot forming temperature and hot formed in the usual manner.
I have recognized that the same maximum property improvement produced by the above-described furnace treatment can be achieved much more quickly, and more economically, and with less total energy. It is achieved by a short induction heating at higher temperatures, as for example 2350.degree. F. for 60 seconds. This recognition is particularly significant because induction heaters are frequently used to reheat presintered powdered metal preforms to hot forming temperatures. These same induction heaters could also be used to presinter lubricant-coated powdered metal preforms. If so, only one heating operation would be required for both presintering and hot forming. This obviously saves energy. Moreover, no separate presintering equipment, preform handling, furnace floor space, and the like would be required. Accordingly, presintering by induction heating is particularly attractive from a commercial production standpoint.
In order to obtain highest quality products, the ferrous powdered metal preforms must be presintered at a temperature above about 2100.degree. F. Moreover, the hot forming lubricant must be applied before heating, to avoid an intermediate cool down for lubricant application between presintering and hot forming. Unfortunately, above about 2050.degree. F., a reaction between the colloidal graphite lubricant and the ferrous powdered metal occurs. This reaction produces surface melting on the ferrous powdered metal preform. Not only does this degrade properties in the resultant article, but it also makes optical monitoring of the preform temperature impossible. When using induction heating, preform temperature is most effectively monitored with an optical pyrometer. Even incipient surface melting changes the optical emissivity of the preform, and the degree of change is erratic. Temperature monitoring of induction heated ferrous powdered metal preform above temperatures of about 2050.degree. F. therefore becomes unreliable. This effectively limits the use of induction heating to lower temperatures, where long heating times are required for maximum presintering effects. This makes induction heating for presintering commerically unattractive, particularly when highest quality articles are desired.
On the other hand, I have found an additive for commercial colloidal graphite lubricants that inhibits the lubricant-ferrous powdered metal preform interaction that ordinarily occurs above about 2050.degree. F. This additive permits one to precoat a preform with lubricant and then presinter above 2100.degree. F. without any noticeable interaction. The lubricant coated preform can now be presintered above 2100.degree. F. during the same heating step, and using the same induction heater, that is now used only to warm a presintered preform up to hot forming temperature after lubricant application. Preforms for high quality ferrous powdered metal articles no longer have to be presintered in a separate and distinct heating operation, cooled, coated with lubricant, and then reheated again to hot forming temperature. I can now combine the two separate heating steps into one operation, to permit sizeable savings in capital investment, time, labor and a variety of ancillary expenses.